1. Field of the Invention
The present invention relates to a method of making a hardened, especially a roll load-bearing, prefabricated steel part and, more particularly, to a method of making a nut for a roll barrel screw drive.
2. Description of the Related Art
Conventionally a semi-finished product from roller bearing steel, for example 100 Cr 6, is pre-shaped in a softened condition during this sort of manufacturing process. Subsequently either the entire structural component or at least its functional surfaces are heat-treated, in order to obtain a surface hardness of greater than or equal to 58 HRC and the hardness depth required for operating under load. Usually a martensitic structure is produced during the heat-treatment, which suffices for the above-mentioned conditions. Dimension and shape changes of the structural component (hardened lagging) result from thermal stresses produced by the heat treatment due to temperature gradients and phase change-related transformation stresses, based on the complex non-single-cause interactions between the internal stresses of the structural components. These dimension and shape changes are for the most part largely stochastic in nature and can only be insufficiently taken into account in the optimization of the starting materials, the softening processing and the heat treatment. The highly accurate geometric shape required for operation of the roll load-bearing structural part, for example the nut of a roll barrel screw drive, can be obtained only by the following procedure. First the part with suitable dimensions is pre-shaped in a softened state. Then in the hardened state it is after-worked with defined or undefined milling or cutting tools in connection with the heat treatment to form the required geometric shape.
Already alternative manufacturing methods have been proposed, which are designed to eliminate the working step in the hardened state. U.S. Pat. No. 6,334,370 B1 should be mentioned for this reason. This starting point however leads to production of structural parts either with insufficient accuracy or to roll load-bearing parts which do not fulfill the general specifications after heat treatment.
A method of making hardened steel parts is disclosed in DE 198 21 797 C1, in which a material characterized as air-hardened steel is used. In this reference, and also in the case of the present invention, the term “air-hardened steel” means a class of materials, whose temperature-time transformation behavior after an austenitization provides cooling speeds, which are sufficient to initiate a martensitic phase transformation, assuming air heat transfer coefficients. In the known method the steel part is heated to over 1100° C. Thermal transformation of the structural part is performed at this temperature and the structural part is subsequently cooled in air and simultaneously subjected to a thermo-mechanical treatment. This method has the disadvantage that the tools used during the thermal transformation and the thermo-mechanical treatment must withstand temperatures of up to 1100° C. Thus the use of special tools is required. Furthermore the working of the component or part must be performed under temperature-controlled conditions, i.e. under spatially and/or engineering limited conditions.